Ultra high frequency converter



Sept. 15, 1942. c. N. KIMBALL ULTRA-HIGH FREQUENCY CONVERTER Filed May 9, 1941 IN'VENTOR CkarleafllKBnall BY kZ ATTORNEY Patented Sept. 15, 1942 UNITED STATES PATENT OFFICE ULTRA HIGH F532; CONVERTER I poration of Delaware Charles N. Kimball, Jackson Heights, N. Y., assignor to Radio Corporation of America, a cor- Application May 9,1941, Serial No. 392,651

3 Claims.

My present invention relates to converter networks and more particularly to a superheterodyne converter stage particularly adapted for reception of signals in the ultra-high frequency range.

As is well known, a superheterodyne converter stage will operate as an amplifier concurrently with its performance as a mixer. Hence, if the signal collector of the receiver has applied to it a signal of intermediate frequency (I. F.) value (that is, a signal whose carrier frequency is equal to the operating I. F. value of the receiver), the

converter tube functioning as an amplifier will give rise to amplified interference signal voltage of the I. F. value. It is highly desirable, therefore, in providing a converter stage for a superheterodyne receiver adapted to operate in the ultra-high frequency range, or for that matter in any other frequency range, to have ideal rejection of undesired signals and particularly those of substantially the I. F. value.

The converter stage should also have minimum ,shot noise. output and maximum signal to noise ratio. In addition, tweets produced by beats between desired signals and the second harmonic of the local oscillator should be minimized. Further, the familiar tweet produced when receiving a signal of twice the I. F. value should be minimized. The triode type of tube is inherently desirable as a converter stage because it has high signal to noise ratio by virtue of its being less prone to produce shot noise. However, the triode possesses a pronounced and highly disadvantageous feedback path from the plate to its control grid. This causes the effective input impedance of the tube to exhibit a resistive loading component in the signal input circuit. At the ultra-high frequencies such input circuit loading is highly undesirable.

Accordingly, it may be stated that it is one of the .main objects of my invention to provide a converter stage for ultra-high frequency signal reception wherein a pair of triodes are arranged to have their control grids connected in parallel to the signal input circuit while their plates are arranged in push-pull relation relative to a common signal output circuit which is tuned to a desired ,1. F. value, the local oscillations being injected in push-pull relation into the common cathode circuit of the tubes, and the control grids normally having biases applied thereto such that they are operated close to plate current cut-off whereby .they have high values of conversion thermore, have the plate to grid feedback paths conductance when operated push pull, and, furautomatically neutralized with the result that the input circuit is free from any loading effect.

Still other objects of my invention are to improve generally the operation of converter networks for ultra-high frequency stages,and more particularly to provide a converter stage which is economically manufactured and assembled in a radio receiver. I

The novel features which I believe to be char-- acteristic of my invention are set forth inpar ticularity in the appended claims; the invention itself, however, as to both its organization and, method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a, circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, there is shown a converter stage employing the present invention. The numeral l denotes a tunable signal input circuit which is tunable over the ultra-high frequency range. The latter may be considered as covering a band of 20 to megacycles (mc.). The input circuit 1 may have coupled thereto any desired type of signal collector such as a di-pole, an ultra-high frequency signal distribution line, or even a grounded antenna circuit. Tubes 2 and 3 are each of the triode type. These can either be separate tubes, or they may be triode sections of a tube of the 6F8G or similar type. Assuming for the sake of simplicity that they are separate tubes, the control grid 4 of tube 2 is connected to the high potential side of input circuit, I, the low potential side thereof being connected to ground. through the high frequency by-pass condenser 5.

The control grid 6 of tube 3 is connected to the lead running to control grid 4, and, hence, signals at the input circuit l are applied in parallel to control grids 4 and 6. The cathode l is connected to the cathode 8 through a coil 9 whose midpoint is connected to ground through the self bias resistor ID. The latter is shunted by a high frequency by-pass condenser II. The resistor Ill has a magnitude such that it develops sufficient direct current voltage thereacross normally to bias grids 4 and 6 close to the plate current cutoff point. I

The plate l2 of tube 2 is connected to the plate l3 of tube 3 through the primary winding M of the I. F. output transformer l5, The condenser l6 shunts winding I4, and tunes the winding to the operating I. F. value. The midpoint of winding I4 is connected to the positive terminal of the direct current energizing source. The secondary circuit of transformer I is tuned to the operating I. F. value, and is designated by numeral Id. The I. F. signal voltage produced in circuit l4 may be impressed on one or more I. F. amplifiers and subsequently detected. The pass band of transformer l5 may be chosen as Wide as is desired depending upon the nature of the signals received. For example, if frequency modulated carrier signals of high carrier frequency deviation ratio are received, then the transformer [5 would have a band width of the order of 200 kilocycles (kc.).

The local oscillator is designated by the numeral 20, and the tunable tank circuit thereof is denoted by numeral 2 I. The variable condensers of circuits l and are shown as having their rotors mechanically controlled in unison, and the dotted line 22 indicates such mechanical unicontrol. The local oscillation energy is applied to coil 9 by coil 9, and the oscillations are applied in push-pull relation to the tubes 2 and 3. The frequency of the local oscillations may be above or below the signal carrier frequency dependent upon the desires of the designer of the receiver. It is well known by those skilled in the art how to maintain the signal and oscillation circuits adjustable over their respective ranges, and yet produce a substantially constant I. F. value in the I. F. output circuit.

The internal plate to grid capacity of each of tubes 2 and 3 is designated by the dotted condensers C prand CgpZ. It will be understood that these are the paths through which feedback normally takes place in the absence of this invention. Those skilled in the art are well acquainted with the Miller effect which causes the effective input impedance of the triode to exhibit a resistive loading component in the input circuit. This resistive loading is caused by the fact that the plate load of the converter is capacitive in effect in so far as the input signal frequency is concerned. The Miller effect is best described as the variation in input impedance in the tube due to feedback through the plate to grid capacity.

In a converter stage the plate load is tuned to the I. F. value, and this may be substantially lower than any signal frequency in the reception band. Hence, to the signal frequency the I. F. circuits appear capacitative, and, therefore, the phase of the converter plate voltage is lagging by approximately 90 degrees with respect to the value it would have if the converter plate load were resonant to the signal frequency. This phase relation of signal grid voltage on the converter tube and its lagging plate voltage causes the input impedance of the tube to contain a shunt resistance component which loads the tuned input circuit. The value of this shunt resistance, which exists effectively from control grid to ground, may be quite low. Its equivalent series effect is, therefore, high. By means of the present invention this loading effect can be entirely removed. By connecting the triodes of the converter so that the plates thereof are in push-pull relation for the I. F. signal currents, as well as for the ultra-high frequency signals themselves, while the signal grids are in cophasal relation for the applied signals, the converter is made self-neutralizing. This relationship is obtained by varying the mutual conductance of the two tubes in a push-pull manner, as by the push-pull injection of the oscillator voltage.

In the absence of this converter arrangement the load of the input circuit is instrumental in reducing the gain and selectivity in the circuit feeding the signal grid of the converter. Hence, it will be possible to use the triodes in a con verter stage with any loading of the input circuit, and yet secure the great advantage of the triode which produces minimum shot noise.

A further advantage of the circuit of this ininvention resides in the fact that the rejection of undesired signals whose frequencies are close to the operating I- F. value, is greatly enhanced. This is due to the fact that the plate circuit of the converter tubes is arranged in push-pull relation, and because the signals are applied to the grids in parallel relation while the oscillations are applied in push-pull relation. By applying the signals and oscillations in this manner there is secured cancellation of the amplifying property of the mixer, but this does not affect the mixing characteristic. By applying the oscillator voltages in push-pull relation theeven order (and zero order) terms in the variation of the mixer mutual conductance with oscillator grid bias during an oscillator cycle are balanced out. There is, therefore, no amplification by the mixer tubes of any signals appearing on the mixer, signal grids. This does not affect, of course, the mixing action of the stage. 7 7 7 Additionally, it will be noted that the two triodes are operated with their control grids biased close to plate current cut-off. The resultant conversion conductance is then approximately equal to one-half; the peak g (mutual conductance) of either tube, as measured at the point of peak oscillator voltage. Thus, {if the oscillator swings the cathodes from cut-off to zero bias, the conversion conductance of the two tubes as a mixer unit is approximately equal to the gm of either tube measured at zero oscillator grid bias divided by 2. In a single tubejconverter, with the oscillator swing and bias adjusted for optimum on (conversion conductance), the best value of ye is about one quarterthe zero bias gm. Hence, there is secured'a '2 to' 1 improvement in conversion conductance with the two tube arrangement. Moreover, the shot noise developed in the mixers is a function of the direct current in the plate circuit during operation. In this two tube circuit the war direct current drawn is less than in a single tubel'converter, for a given value of conversion conductance developed in the converter unit. Hence, the signal to noise ratio is greatly improved by using this arrangement. 7 I

Finally, since there are no even harmonics at the oscillator frequency in the mixer plate circuits, tweets produced by beats between signals and the second harmonic of the oscillator are reduced. Also, the familiar ftweet produced when receiving a signal of twice the II Fffrequency value is reduced. i

Preferably, the triodes should be chosen so that they have substantially the same'gain and substantially the same value of plateto grid capacity. In this case the current through one of the inherent capacities from the plate of one tube to its grid is equal and opposite to that from the plate of thesecond tube to the common signal grids, and the system is inherently neutralized.

It is to be understood that the usual automatic volume control (AVG) may be applied to the control grids 4 and 6. It is not'believed necessary to show the details of an AVC network, there being merely shown the AVG lead including the usual pulsation voltage filter resistor 3|. It will be understood that AVC lead is connected to a point on a rectifier load resistor which assumes an increasingly negative potential with respect to ground as the carrier amplitude increases. Such rectifier may be supplied with signal carrier energy from any portion of the I. F. network in the usual and well known manner. AVC bias may be also applied to the I. F. amplifiers as indicated in the drawing.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made Without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. A heterodyne receiver circuit comprising a converter stage consisting of a pair of triodes of substantially equal gain, a common signal input circuit tuned to resonate at the input signal frequency, said input circuit being coupled to the control grids of said triodes to apply signals thereto in parallel, means for connecting the plates of said triodes in push-pull relation to provide a common output circuit tuned to a desired intermediate frequency value, a local oscillator tuned to a frequency differing from the input circuit frequency by said intermediate frequency, means for impressing oscillations from said oscillator upon the cathodes of said triodes in push-pull relation, and means common to the space current paths of both triodes for providing a normal bias voltage for said control grids such as substantially to cut oil plate current in the absence of received signals thereby to pro- -yide max mum conversion conductance, each of ;said triodeshaving substantially equal inherent plate to grid capacity sufficient to provide input ,circlu't loading in the absence of the other triode, said input circuit loading being prevented by virtue of the automatic neutralization of the aforesaid connections.

2. In (combination with a triode consisting of :a cathode, a control grid and a plate, a resonant input circuit tuned to a high signal frequency, ;means connecting said input circuit between the ;grid and cathode of said triode, a resonant output circuit connected to the plate of said triode, said triode having inherent plate to grid capacity sufiicient to provide feedback of such magnitude that substantial input circuit loading results, a second triode having characteristics similar to the first triode and having inherent plate to grid capacity of substantially the same magnitude as the similar capacity of said first triode, meanspconnecting the grid of the second triode to said input circuit whereby the second triode grid receives signals therefrom in parallel with the first triode grid, means connecting the plate of the second triode to said first triode output circuit and in push-pull relation thereto, said output circuit being tuned to a desired intermediate frequency value, means connecting the cathode of the second triode with the cathode of the first triode, a local oscillator coupled to both cathodes so as to apply local oscillations thereto in push-pull relation, said local oscillator being tuned to a frequency which differs from, the signal frequency by said intermediate frequency, and said input circuit loading being effectively eliminated by virtue of the neutralization due to said connections.

3. A heterodyne receiver circuit comprising a converter stage consisting of a pair of triodes of substantially equal gain, a signal input circuit tuned to a frequency of the order of 20 to megacycles, said input circuit being coupled to the control grids of said triodes to apply signals thereto in parallel, means for connecting the plates of said triodes in push-pull relation to provide an output circuit tuned to a desired intermediate frequency value, a local oscillator CHARLES N. KIIWBALL. 

